Method for adjusting the particle size of popcorn polymers during popcorn polymerization

ABSTRACT

The particle size of popcorn polymers is established during the popcorn polymerization by a process in which reaction mixtures forming popcorn polymers and comprising monoethylenically unsaturated monomers and a crosslinking agent are polymerized in the absence of oxygen and polymerization initiators at up to 200° C. and in which the popcorn polymerization is carried out as a precipitation polymerization in water or in the absence of a solvent and the particle size of the popcorn polymers is controlled in the range from 1 μm to 10 mm by passing an inert gas stream into the reaction mixture, and popcorn polymers having a mean particle diameter of from &gt;400 μm to 1500 μm are used as beverage clarifiers, antidiarrheal drugs and disintegration accelerators for tablets.

The present invention relates to a process for establishing the particlesize of popcorn polymers during popcorn polymerization, reactionmixtures forming popcorn polymers and comprising monoethylenicallyunsaturated monomers and a crosslinking agent being polymerized in theabsence of oxygen and polymerization initiators at up to 200° C.

It is known that the homopolymers of N-vinylpyrrolidone are generallyreadily soluble in water and in numerous organic solvents. Furthermore,it is known that insoluble, more or less swellable copolymers based onN-vinylpyrrolidone can be prepared by incorporating at leastbifunctional vinyl or acryloyl compounds, which act as crosslinkingagents, as polymerized units in a conventional manner. However, evenwhen relatively large amounts of bifunctional components are used, it isnot possible to prepare polymers having only low swellability andcontrollable particle size in water. Polymers based onN-vinylpyrrolidone and having low swellability are prepared, forexample, by popcorn polymerization, cf. for example DE-A-2059484,DE-A-2255263 and U.S. Pat. No. 3,277,066.

U.S. Pat. No. 4,451,582 discloses a process for the preparation ofinsoluble, granular polymers which are only slightly swellable in waterand comprise basic vinyl heterocycles having a pKa value of more than 4and their copolymers with up to 30% by weight of copolymerizablemonomers, in which the monomers are polymerized in the presence of from0.1 to 10% by weight, based on the total amount of monomers, ofcrosslinking agent in the absence of oxygen and polymerizationinitiators. The polymers are preferably prepared by precipitationpolymerization in water. However, they can also be obtained in theabsence of solvents for the monomers, by heating the monomers totemperatures of preferably from 150 to 180° C. This reaction is howeverpoorly controllable and gives only low space-time yields and relativelyhighly contaminated products.

It is also known that insoluble, only slightly swellable polymers ofacrylic acid can be prepared by polymerizing acrylic acid in aqueoussolution in the presence of small amounts of2,2′-azobisisobutyronitrile, cf. J. W. Breitenbach et al., Makromol.Chemie 177, (1976), 2787 to 2792. However, this gives polymer blendswhich contain a glassy polymer (gel) as the main component and popcornpolymers of acrylic acid in minor amounts. The separation of the polymerblends cannot be carried out at an acceptable cost in industry.

Houben-Weyl, Volume 14, Makrom. Stoffe, Part 1 (1961), page 98,discloses that popcorn polymers are formed from mixtures of styrene withdivinylbenzene. Popcorn polymers also form in the industrialbutadiene/styrene polymerization. Methyl acrylate, too, tends to formpopcorn polymers. On contact with the monomers of which they consist orwith other monomers, popcorn polymers have the property of being able toconvert them into popcorn polymers. They act as a nucleus for thepolymerization. However, the activity is lost if they come into contactwith oxygen.

Ullmanns Encyklopädie der Techn. Chemie, 4^(th) Edition, Volume 19,1980, page 385, discloses that an insoluble polymer slightly swellablein water is formed on heating N-vinylpyrrolidone with hydroxides andalcoholates of alkali metals and alkaline earth metals. Such substancesreferred to as popcorn polymers also form on heating N-vinylpyrrolidonewith divinyl compounds in the absence of oxygen and polymerizationinitiators. In the absence of bases as a rule several hours are requiredbefore the popcorn polymerization begins at all.

EP-B-0 177 812 discloses a process for the preparation of insoluble,only slightly swellable, polymer powders based on monoethylenicallyunsaturated carboxamides, carboxylic acids and carboxylic esters, inwhich the polymerization is initiated using an active popcorn polymerwhich is obtainable by heating a mixture of from 99.6 to 98.8% by weightof N-vinylpyrrolid-2-one and from 0.4 to 1.2% by weight of a compoundhaving at least two ethylenically unsaturated double bonds, as acrosslinking agent, to a temperature of from 100 to 150° C. in theabsence of oxygen and polymerization initiators. As soon as the activepopcorn polymer has formed, a mixture of

a) from 99.9 to 90% by weight of at least one compound from the groupconsisting of the N-vinylcarboxamides, acrylamide, methacrylamide,acrylic acid, methacrylic acid, acrylic esters, methacrylic estersand/or vinyl esters and

b) from 0.1 to 10% by weight, based on the total monomers, of a compoundhaving at least two ethylenically unsaturated double bonds, as acrosslinking agent,

is polymerized therein in a powder bed in the presence of a heattransfer agent which is inert to the reactants, at from 90 to 220° C.while maintaining the powder state, circulating the reaction materialand evaporating the heat transfer agent from the polymerization zone, inthe absence of oxygen and polymerization initiators. The popcornpolymers thus obtainable are used, for example, for absorbing tanninsfrom beverages, as dissolution accelerators for pharmaceutical tabletsand as ion exchangers. Carboxyl-containing popcorn polymers areobtained, for example, by hydrolyzing popcorn polymers containingacrylic ester and acrylamide units.

WO-A-94/11408 discloses insoluble, only slightly swellable polymershaving amino groups. These polymers are prepared by polymerizingopen-chain N-vinylcarboxamides, such as N-vinylformamide, and, ifrequired, other monomers copolymerizable therewith with from 0.1 to 10%by weight, based on the monomers used in the polymerization, of acompound having at least two ethylenically unsaturated double bonds, asa crosslinking agent, in the absence of oxygen and polymerizationinitiators to give popcorn polymers and eliminating the formyl or acylgroup from at least 2% of the N-vinylcarboxamides incorporated aspolymerized units, with formation of amino-containing popcorn polymers,by the action of enzymes, bases or acids. The popcorn polymers are usedas ion exchangers or as adsorber resin for metal ions.

The disadvantage of said processes for the preparation of popcornpolymers is that any desired particle size of the polymer material isformed more or less accidentally and can be brought to the particle sizerequired for the particular application only in an inconvenient mannerby mechanical comminution or by agglomeration. However, it is essentialto establish the particle size for many applications.

It is an object of the present invention to provide a process forestablishing the particle size of popcorn polymers during the popcornpolymerization.

We have found that this object is achieved, according to the invention,by a process for establishing the particle size of popcorn polymersduring the popcorn polymerization, reaction mixtures forming popcornpolymers and comprising monoethylenically unsaturated monomers and acrosslinking agent being polymerized in the absence of oxygen andpolymerization initiators at up to 200° C., if the popcornpolymerization is carried out as a precipitation polymerization in wateror in the absence of a solvent and the particle size of the popcornpolymers is controlled in the range from 1 μm to 10 mm by passing aninert gas stream into the reaction mixture. For example, from 0.01 to100, preferably from 0.05 to 30, in particular from 0.1 to 20, liters ofinert gas per hour per liter of reaction mixture are required for thispurpose.

For the preparation of popcorn polymers, for example,

a) N-vinyllactams of the formula

where n is 3-5, and, if required,

b) other monoethylenically unsaturated monomers copolymerizabletherewith

are polymerized in the presence of

c) from 0.1 to 10% by weight, based on the monomers a) and b) used inthe polymerization, of a compound containing at least two ethylenicallyunsaturated double bonds, as a crosslinking agent, in the absence ofpolymerization initiators and oxygen to give popcorn polymers.

Suitable compounds of the formula I are, for example,N-vinylpyrrolidone, N-vinylpiperidone and N-vinylcaprolactam. In 30 thepolymerization, it is also possible to use mixtures of the monomers ofgroup (a), for example mixtures of N-vinylpyrrolidone andN-vinylcaprolactam.

The monomers of group (b), which, if required, is present in thepreparation of the popcorn polymers, are other monoethylenicallyunsaturated compounds copolymerizable with the monomers of groups (a)and (c). These include, for example, acrylamide, methacrylamide, acrylicacid, methacrylic acid, acrylic esters, methacrylic esters and/or vinylesters. The acrylic and methacrylic esters are preferably derived fromsaturated, monohydric alcohols of 1 to 4 carbon atoms and saturateddihydric alcohols of 2 to 4 carbon atoms, respectively. Examples ofthese esters are methyl acrylate, methyl methacrylate, ethyl acrylate,ethyl methacrylate, n-propyl acrylate, n-propyl methacrylate, isopropylacrylate, isopropyl methacrylate and the esters of acrylic acid andmethacrylic acid which are derived from the isomeric butanols, andhydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropylacrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate,hydroxyisobutyl acrylate and hydroxyisobutyl methacrylate. Among thevinyl esters, vinyl acetate and vinyl propionate are preferred. Furthersuitable monomers of group (b) are styrene, p-tert-butylstyrene,acrylonitrile, methacrylonitrile, N-vinylformamide,N-vinyl-N-methylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide,N-vinyl-N-ethylformamide, N-vinyl-N-(n-propyl)formamide,N-vinyl-N-isopropylformamide, N-vinyl-N-isobutylformamide,N-vinyl-N-methylpropionamide, N-vinyl-n-butylacetamide andN-vinyl-N-methylpropionamide, 1-vinylimidazole,2-methyl-1-vinylimidazole and 4-methyl-1-vinylimidazole.

The monomers of group (b), singly or as a mixture with one another, canbe polymerized together with the monomers of groups (a) and (c). Thepopcorn polymerization can be initiated, for example, by heatingN-vinylpyrrolidone and small amounts, for example from 0.4 to 1.2% byweight, of a crosslinking agent, such as N,N′-divinylethyleneurea, in anaqueous medium in the presence of an alkali. Freshly distilledN-vinylpyrrolidone is preferably used for initiating the popcornpolymerization.

The popcorn polymerization takes place particularly readily withN-vinylpyrrolidone at from about 60 to 150° C. in the absence of oxygenand polymerization initiators.

If they are concomitantly used in the preparation of the popcornpolymers, the monomers of group (b) are present in an amount of 30 from0.1 to 99.9% by weight in the monomer mixture comprising (a) and (b).

Compounds which contain at least two ethylenically unsaturated doublebonds in the molecule are used as monomers of group (c), as crosslinkingagents, in the polymerization. For example, alkylenebisacrylamides, suchas methylenebisacrylamide, and N,N′-acryloylethylenediamine,N,N′-divinylethyleneurea, N,N′-divinylpropyleneurea,ethylidenebis-3-(N-vinylpyrrolidone),N,N′-divinyl-2,2′-diimidazolylbutane and1,1′-bis-(3,3′-vinylbenzimidazolyl-2-one)-1,4-butane, are particularlysuitable. Other suitable crosslinking agents are, for example, alkyleneglycol di(meth)acrylates, such as ethylene glycol diacrylate, ethyleneglycol dimethacrylate, tetraethylene glycol acrylate, tetraethyleneglycol dimethacrylate, diethylene glycol acrylate and diethylene glycolmethacrylate, aromatic divinyl compounds, such as divinylbenzene anddivinyltoluene, and vinyl acrylate, allyl acrylate, allyl methacrylate,divinyldioxane, pentaerythrityl triallyl ether and mixtures of thecrosslinking agents. The crosslinking agents are used in amounts of from0.1 to 10, preferably from 1 to 4, % by weight, based on the monomers(a) and (b) used in the polymerization.

The popcorn polymerization is carried out by known methods, for exampleas precipitation polymerization or by mass polymerization. For example,a procedure is known in which—as described in EP-B-0 177 812—the popcornpolymerization is initiated by heating a mixture of from 99.6 to 98.8%by weight of N-vinylpyrrolidone and from 0.4 to 1.2% by weight of acompound having at least two ethylenically unsaturated double bonds, asa crosslinking agent, to a temperature of from 100 to 150° C. in theabsence of oxygen and polymerization initiators. This polymerization ispreferably initiated by the presence of small amounts of sodiumhydroxide solution or potassium hydroxide solution. Within a short time,a polymerizable popcorn polymer forms and, on addition of suitable othermonomer mixtures, i.e. the monomers of group (a) and, if required, (b)and further addition of the monomers (c), initiates and completes thepopcorn polymerization of these monomers without an induction period,according to the invention an inert gas stream preferably being passed,before the beginning of the popcorn polymerization, into the monomersinitially taken in the polymerization vessel, for controlling theparticle size of the resulting popcorn polymers. However, theintroduction of the inert gas stream is begun during the inductionperiod of the polymerization at the latest, the inert gas stream beingpassed through the reaction mixture during the total duration of thepopcorn polymerization.

To carry out the popcorn polymerization without a solvent, i.e. as amass polymerization, the monomer mixture comprising (a) and (c) and, ifrequired, (b) is rendered inert by passing in nitrogen and is thenheated to a temperature of from 100 to 200° C., preferably from 150 to180° C. It is advantageous furthermore to pass a gentle stream ofnitrogen through the monomers also during the polymerization. Theexclusion of oxygen is also achieved by polymerizing the batch at apressure which is below atmospheric pressure and at which the monomersboil. However, the popcorn polymerization can be carried out underreduced pressure and while simultaneously passing in an inert gas.Depending on the type of monomers used and on the chosen temperature,the mixture is polymerized in the course of from 1 to 20 hours. Forexample, in the polymerization of N-vinylcarboxamides with 2% by weightof N,N′-divinylethyleneurea at 150° C. while stirring with a powerfulstirrer and at 310 mbar and with the introduction of nitrogen, the firstpolymer particles form after 2.5 hours and their amounts slowly increaseuntil the reaction mixture consists of a powder after a polymerizationtime of 10 hours. The popcorn polymer is obtained therefrom in yields ofmore than 90% in the form of a powder having an average particle size offrom about 10 μm to 5 mm, preferably from more than 400 μm to 1000 μm.

For the preparation of the popcorn polymers, however, the precipitationpolymerization in water is preferred. The concentration of the monomersis expediently chosen so that the reaction mixture can be thoroughlystirred over the total reaction time. If the concentration of themonomers in water is too high, e.g. 95% by weight, the polymer particlesbecome tacky, so that stirring becomes more difficult than in moredilute aqueous solution. To carry out the reaction in the conventionalstirred kettles, for example, monomer concentrations of from about 5 to30, preferably from 10 to 20, % by weight, based on the aqueous mixture,are chosen. If more powerful stirrers are available, the monomerconcentration of the aqueous solution can also be increased to 50% byweight, if required even higher. In some cases, it may be expedient tobegin the popcorn polymerization with a relatively concentrated solutionand then to add water for dilution in the course of the reaction.

The popcorn polymerization is preferably carried out at a pH above 6, inorder to avoid possible hydrolysis of the monomers. The pH can beestablished by adding small amounts of bases, such as sodium hydroxideor ammonia, or of the conventional buffer salts, such as sodiumcarbonate, sodium bicarbonate or sodium phosphate. If required, theexclusion of oxygen can be achieved by heating the mixture to bepolymerized to the boil and additionally passing an inert gas streamthrough the reaction mixture for controlling the particle size of thepopcorn polymers.

The particle size of the popcorn polymers is controlled by the amount ofthe inert gas stream which is passed through the polymerizationsolution. If a little inert gas is fed to the reaction batch, as a rulecoarse popcorn polymers form; if a large amount of inert gas flowsthrough the polymerization solution, finer popcorn polymers form.Depending on various boundary conditions, for example kettle size,temperature and pressure, it is not possible to state exact conditionsfor establishing a specific particle size of the popcorn polymers forthe respective individual case. The correct amount of inert gas for theindividual case, which has to be passed through the reaction mixture,can be readily determined by a few simple experiments. As stated above,the amounts of inert gas which are to be passed through the reactionmixture are from 0.01 to 100, preferably from 0.05 to 30, l of inert gasper 1 of reaction mixture per h.

The inert gas used may be a noble gas, such as helium, neon or argon.Carbon dioxide is also suitable. Nitrogen is preferably used.

In some cases, it may also be advantageous to add small amounts—e.g.from 0.1 to 1% by weight, based on the monomer mixture—of a reducingagent, such as sodium sulfite, sodium pyrosulfite, sodium dithionite,ascorbic acid or mixtures of the reducing agents, to the reactionmixture for complete removal of dissolved oxygen.

The polymerization temperature may be varied within a wide range, forexample from about 20 to 200° C., preferably from 50 to 150° C.

In a particularly preferred embodiment of the precipitationpolymerization, a water-soluble comonomer, some of the crosslinkingagent, water and, if required, a buffer and a reducing agent are heatedin an inert gas stream until the first polymer particles appear. Then—ifdesired—a mixture rendered inert beforehand by blowing in nitrogen andcomprising one or more of the abovementioned comonomers and theremaining crosslinking agent and, if required, water as a diluent isadded in the course of from 0.2 to 5 hours. This procedure has theadvantage that the popcorn polymerization takes only a short time.

The popcorn polymers can be isolated from the aqueous solution, purifiedand then—if the monomers incorporated as polymerized units containhydrolyzable groups, e.g. monomers such as vinylformamide or methylmethacrylate—hydrolyzed. However, it is also possible to hydrolyze theaqueous suspension of the popcorn polymers directly.

The popcorn polymers are usually obtained in a yield of from about 90to >99% of the theoretical yield. They can be isolated from the aqueoussuspension by filtration or centrifuging with subsequent washing outwith water and drying in conventional dryers, such as athrough-circulation dryer, a vacuum dryer, a paddle dryer, a tumblerdryer, a fluidized-bed dryer or a pneumatic dryer. The popcorn polymersare virtually insoluble in water and all solvents and swell onlyslightly therein. The mean particle diameter of the dried popcornpolymers is usually from 1 μm to 10 mm. For the preparation of popcornpolymers, N-vinylpyrrolidone, N-vinylpiperidone, N-vinylcaprolactam or amixture thereof is preferably used as monomers forming popcorn polymers.The preferably used crosslinking agent is N,N′-divinylethyleneurea.

Popcorn polymers having a mean particle diameter of from >400 μM to 1500μm are used, for example, as beverage clarifiers, antidiarrheal drugsand disintegration accelerators for tablets.

Of particular interest is the use of popcorn polymers having a meanparticle diameter of from 420 to 1000 μm in an amount of from 0.5 to 20%by weight in detergent and cleaning agent tablets for increasing thedisintegration rate of the tablets on combination with water.

In the examples which follow, percentages are by weight. The particlesize distribution of the popcorn polymers was determined by sieveanalysis.

EXAMPLES Comparative Example 1

1375 g of distilled water, 115 g of N-vinylpyrrolidone, 2.6 g ofN,N′-divinylethyleneurea and 0.05 g of sodium hydroxide were initiallytaken in a stirred apparatus and heated to 60° C. while stirring at 60rpm under a stream of nitrogen, which however was passed not through thereaction mixture but only above it. As soon as this temperature had beenreached, 130 mg of sodium dithionite were added. The mixture was kept at60° C. The popcorn polymerization took place very slowly and wascomplete after about 24 hours. The viscous suspension was filtered andthe residue was washed with water (about 2000 ml) in order to removeimpurities, such as soluble polymer and unpolymerized monomers. Theyield of popcorn polymer was 100%.

Sieve Analysis:

Particle size [μm] Fraction [%] >500 97.6 250-500 2.4 <250 0

The comparative example was repeated twice. In each case, the particlesize distribution of the popcorn polymers obtained was determined. Theresults are shown in the table below:

Comparative Example 2 3 Particle size [μm] Fraction [%] [%] >500 80.571.2 250-500 14.9 19.5 <250 4.6 9.3

The comparative examples show that the particle size of the popcornpolymers according to the prior art was not reproducible.

Example 1a

1375 g of distilled water, 115 g of N-vinylpyrrolidone, 2.6 g ofN,N′-divinylethyleneurea and 0.05 g of sodium hydroxide were initiallytaken in a stirred apparatus and heated to 60° C. while stirring at aspeed of 60 rpm. Nitrogen, which was fed into the reaction mixture withthe aid of a tube which extended to the bottom of the stirred apparatus,flowed through the solution as early as during the heating-up perioduntil completion of the polymerization. The flow rate was 6 l/h. Afterthe temperature of the reaction mixture had reached 60° C., 130 mg ofsodium dithionite were added. The mixture was kept at 60° C. The popcornpolymerization began after about 20 minutes and was complete after 5hours. The viscous suspension was then filtered and the residue waswashed with water (about 2000 ml) in order to remove impurities such assoluble polymer and residual monomers. The yield of popcorn polymer was97%.

Example 1b

Example 1a was repeated exactly. The particle size distribution wasdetermined. It is shown in the table below. As can be seen there, theparticle distribution of the popcorn polymers differs only slightly fromthat of Example 1a. In contrast with the prior art processes, theresults are reproducible.

Sieve Analysis:

Example 1 a) b) Particle size [mm] Fraction [%] Fraction [%] >500 72.074.3 250-500 13.6 15.5 <250 14.4 10.2

Example 2

1375 g of distilled water, 115 g of N-vinylpyrrolidone, 2.6 g ofN,N′-divinylethyleneurea and 0.05 g of sodium hydroxide were initiallytaken in a stirred apparatus and heated to 60° C. while stirring at aspeed of 60 rpm. Nitrogen was passed from below into the reactionmixture and flowed through the solution during heating-up to the end ofthe reaction. The flow rate was 18 l/h. After the reaction mixture hadreached 60° C., 26 mg of sodium dithionite were added. The mixture waskept at 60° C. The popcorn polymerization began after about 20 minutesand was complete after 5.5 hours. The viscous suspension was filteredand the residue was washed with water (about 2000 ml) in order to removeimpurities such as soluble polymer and residual monomers. The yield ofpopcorn polymer was 100%.

Sieve Analysis:

Particle size [μm] Fraction [%] >500 79.1 250-500 15.3 <250 5.6

Example 3

1375 g of distilled water, 115 g of N-vinylpyrrolidone, 2.6 g ofN,N′-divinylethyleneurea and 0.05 g of sodium hydroxide were initiallytaken in a stirred apparatus and heated to 60° C. while stirring at aspeed of 60 rpm. Nitrogen flowed through the solution during theheating-up and the polymerization. The nitrogen was passed into thereaction mixture with the aid of a tube which extended to the bottom ofthe stirred apparatus. The flow rate was 6 l/h. After the reactionmixture had reached 60° C., 26 mg of sodium dithionite were added. Themixture was kept at 60° C. and stirred continuously. The popcornpolymerization began after about 20 minutes and was complete after about6 hours. The viscous suspension was filtered and the residue was washedwith water (about 2000 ml) in order to remove impurities such as solublepolymer and residual monomers. The yield of popcorn 40 polymer was 99%.

Sieve Analysis:

Particle size [μm] Fraction [%] >500 94.8 250-500 13.0 <250 9.2

Example 4

1375 g of distilled water, 115 g of N-vinylpyrrolidone, 2.6 g ofN,N′-divinylethyleneurea and 0.05 g of sodium hydroxide were initiallytaken in a stirred apparatus and heated to 60° C. while stirring at aspeed of 60 rpm. Nitrogen flowed through the solution from the beginningof heating-up to the end of the polymerization. The nitrogen was passedinto the reaction mixture with the aid of a tube which ended at thebottom of the stirred apparatus. The flow rate of the nitrogen was 18l/h. After the temperature of the reaction mixture had reached 60° C.,260 mg of sodium dithionite were added. The mixture was kept at 60° C.and stirred continuously. The popcorn polymerization began after about20 minutes and was complete after about 5 hours. The viscous suspensionwas filtered and the residue was washed with water (about 2000 ml) inorder to remove impurities such as soluble polymer and residualmonomers. The yield of popcorn polymer was 98%.

Sieve Analysis:

Particle size [μm] Fraction [%] >500 0 250-500 23.3 <250 76.7

Example 5

1375 g of distilled water, 115 g of N-vinylpyrrolidone, 2.6 g ofN,N′-divinylethyleneurea and 0.05 g of sodium hydroxide were initiallytaken in a stirred apparatus and heated to 60° C. while stirring at aspeed of 60 rpm. Nitrogen was passed into the reaction mixture at thebottom of the stirred apparatus and flowed through the solution. Theflow rate was 6 l/h. After the temperature of the reaction mixture hadreached 60° C., 260 mg of sodium dithionite were added. The mixture waskept at 60° C. and stirred continuously. The popcorn polymerizationbegan after about 20 minutes and was complete after about 7 hours. Theviscous suspension was filtered and the residue was washed with water(about 2000 ml) in order to remove impurities such as soluble polymerand residual monomers. The yield of popcorn polymer was 93%.

Sieve Analysis:

Particle size [μm] Fraction [%] >500 82.6 250-500 16.3 <250 10.4

Example 6

1375 kg of distilled water, 115 kg of N-vinylpyrrolidone, 2.6 kg ofN,N′-divinylethyleneurea and 1 kg of 5% strength sodium hydroxidesolution were initially taken in a stirred kettle and heated to 75° C.while stirring at a rate of 60 rpm, nitrogen, which was passed into thereaction mixture at the bottom of the polymerization vessel, flowingthrough the solution during the heating-up and the polymerization. Theflow rate was 350 l/h. After the temperature of the reaction mixture hadincreased to 75° C., 26 g of sodium dithionite, dissolved in 30 ml ofwater, were added. The mixture was kept at 75° C. and stirredcontinuously. The popcorn polymerization began after about 20 minutesand was complete after 6 hours. The viscous suspension was then filteredand the residue was washed with water in order to remove impurities suchas soluble polymer and unpolymerized monomers. The yield of popcornpolymer was 94%.

TABLE 1 Sodium dithionite Sieve analysis Example [mg] N₂ [l/h] >500 [μm]250-500 [μm] <250 [μm] 1 130 6 72.0 13.6 14.4 2  26 18 79.1 15.3 5.6 3 26 6 94.8 13.0 9.2 4 260 18 0 23.3 76.7 5 260 6 82.5 16.3 10.4 6 26 g350 88.2 10.2 1.6

As shown by Examples 1 to 6, an unexpected increase in thepolymerization rate is obtained according to the invention compared withthe procedure of Comparative Example 1 (no nitrogen passed into thereaction mixture).

To demonstrate the mode of action of popcorn polymers as disintegrationaccelerators for detergent tablets, detergent tablets weighing about 4 gand having a diameter of 2.5 cm were produced from the detergentformulation A stated below and the popcorn polymers stated in Table 2.Tabletting was carried out in an eccentric press (from Korsch, type EK 0DMS). The mixtures contained, as a disintegrant which was mixed with thepulverulent granulated detergent powder, the amounts of popcorn polymerwhich are stated in the table.

A disintegration tester from Erweka was used for determining thesolubility. For this purpose, the tablets were agitated in adisintegration test basket with sieve base in a thermostatted water bath(30 upward and downward movements per minute over a height of 20 mm permovement). The time after which residue was no longer present on thesieve base was determined. The results are shown in the table.

Detergent of composition A (all data in % by weight)Alkylbenzenesulfonate 8 Potassium coconut oil soap 1.2 Nonionicsurfactant 6 (1 mol of C₁₃/C₁₅-fatty alcohol, reacted with 7 mol ofethylene oxide) Zeolite A 35 Sodium carbonate 8 Sodium metasilicate.5.5H₂O 6 Sodium citrate 4 Sodium percarbonate 18 Tetraacetylethylenediamine5 (TAED) Complexing agent 0.3 (Ethylenediaminetetra- methylenephosphonate) 70:30 acrylic acid/ 4 maleic acid copolymer, molar mass70,000 Sodium sulfate 3 Other constituents 2.5 (fragrances, antifoams,enzymes, optical brighteners) Bulk density about 770 g/l Particle sizeabout 700-1000 μm

TABLE 2 Mean Tabletting Dissolu- Exam- particle Compression pressuretion ple Detergent A size [μm] force kN MPa time (a) 2% Polymer  600 8.614.5 <30 sec. A¹⁾ (b) 1.5% Polymer >400 14.5 <30 sec. A (c) Without 4.0913 30 min polymer (d) Polymer B²⁾ <100 5.98 15 min ¹⁾Prepared accordingto Example 6 ²⁾Commercial popcorn polymer based on polyvinylpyrrolidone

We claim:
 1. A process comprising, polymerizing a reaction mixturecomprising at least one monoethylenically unsaturated monomer and acrosslinking agent in the absence of oxygen and polymerizationinhibitors, while passing an inert gas stream into the reaction mixtureto form one or more popcorn polymers having a particle size in a rangenarrower and within the range of from 1 μm to 10 mm, controlling theamount of the inert gas stream passed into the reaction mixture in orderto obtain said polymers having a particle size in said narrower range,wherein said polymerizing is a precipitation polymerization in water orin the absence of a solvent at a temperature of up to 200° C.
 2. Theprocess as claimed in claim 1, wherein the amount of inert gas passedinto the reaction mixture is from 0.01 to 100 l of inert gas per hourper l of the reaction mixture.
 3. The process as claimed in claim 1,wherein the polymerization is carried out in the presence of a reducingagent.
 4. The process as claimed in claim 3, further comprising addingthe reducing agent to the reaction mixture after beginning passing theinert gas.
 5. The process as claimed in claim 1, wherein the inert gasis nitrogen.
 6. The process as claimed in claim 1, wherein themonoethylenically unsaturated monomers are N-vinyllactams.
 7. Theprocess as claimed in claim 1, wherein the monomers are selected fromthe group consisting of N-vinylpyrrolidone, N-vinylpiperidone,N-vinylcaprolactam and mixtures thereof.
 8. The process as claimed inclaim 1, wherein the crosslinking agent is N, N′-divinylethyleneurea. 9.A popcorn polymer produced by the process as claimed in claim 1, whereinthe popcorn polymer has a mean particle diameter of from greater than400 μm to 1500 μm.
 10. A composition comprising from 0.5 to 20% byweight of the popcorn polymer claimed in claim 9, wherein the popcornpolymer has a mean particle diameter of from 420 μm to 1000 μm.
 11. Amethod of clarifying a beverage comprising, bringing into contact thebeverage with a popcorn polymer produced by the process claimed in claim1, wherein the popcorn polymer has a mean particle diameter of from 420μm to 1000 μm.
 12. An antidiarrheal drug comprising popcorn polymerproduced by the process as claimed in claim 1, wherein the popcornpolymer has a mean particle diameter of from greater than 400 μm to 1500μm.
 13. A tablet comprising a popcorn polymer produced by the process asclaimed in claim 1, wherein the popcorn polymer has a mean particlediameter of from greater than 400 μm to 1500 μm, wherein the popcornpolymer is capable of accelerating the disintegration of the tablet. 14.A detergent tablet comprising from 0.5 to 20% by weight of a popcornpolymer produced by the process as claimed in claim 1, wherein thepopcorn polymer has a mean particle diameter of from greater than 400 μmto 1500 μm and wherein the popcorn polymer is capable of increasing thedisintegration rate of the tablet when mixed with water.